FIG. 11 shows a fluid quality sensor of WO2007/019577. In order that a fluid S flowing from an upstream fluid pipe 25 toward a downstream fluid pipe 27 may be detected based on electrostatic capacitance and temperature, a pipe 61 connected to the fluid pipe 25, a pipe 62 connected to the fluid pipe 27, and an intermediate pipe 63 interposed between the pipes 61 and 62 form a fluid path 64. An internal electrode 65 is disposed on the fluid path 64 inside the immediate pipe 63 via a supporting member 66. The intermediate pipe 63 is an external electrode, and the internal electrode 65 is electrically insulated from the immediate pipe 63 by the supporting member 66. A temperature sensor 67 indirectly detects the temperature of the internal electrode 65 with a detector 68 thermally connected to the supporting member 66.
Thus, based on a capacitance value between the intermediate pipe 63 and the internal electrode 65 and a temperature detected by the temperature sensor 67, the state such as the concentration of the fluid is detected.
Further, FIG. 12 shows a fluid quality sensor of Japanese Patent No. 2639753.
Pipes 70 and 71 are joined to each other in a substantially T-shape. A fluid S flows in from an inlet 70a on one end of the pipe 70 and flows out from an outlet 71a on one end of the pipe 71. The other end of the pipe 70 is closed by an electrode inserting member 72. An internal electrode 73 is inserted from the other end toward the one end of the pipe 70. Reference numeral 74 denotes a temperature sensor attached to the outer peripheral surface of the pipe 70.
Thus, based on a capacitance value between the pipe 70 and the internal electrode 73 and a temperature detected by the temperature sensor 74, the state such as the concentration of the fluid S is detected.